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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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76 </div>
77 <hr>
78 <a name="Wisdom-String-Export_002fImport-from-Fortran-1"></a>
79 <h4 class="subsection">7.6.2 Wisdom String Export/Import from Fortran</h4>
80
81 <a name="index-fftw_005fexport_005fwisdom_005fto_005fstring-1"></a>
82 <p>Dealing with FFTW&rsquo;s C string export/import is a bit more painful. In
83 particular, the <code>fftw_export_wisdom_to_string</code> function requires
84 you to deal with a dynamically allocated C string. To get its length,
85 you must define an interface to the C <code>strlen</code> function, and to
86 deallocate it you must define an interface to C <code>free</code>:
87 </p>
88 <div class="example">
89 <pre class="example"> use, intrinsic :: iso_c_binding
90 interface
91 integer(C_INT) function strlen(s) bind(C, name='strlen')
92 import
93 type(C_PTR), value :: s
94 end function strlen
95 subroutine free(p) bind(C, name='free')
96 import
97 type(C_PTR), value :: p
98 end subroutine free
99 end interface
100 </pre></div>
101
102 <p>Given these definitions, you can then export wisdom to a Fortran
103 character array:
104 </p>
105 <div class="example">
106 <pre class="example"> character(C_CHAR), pointer :: s(:)
107 integer(C_SIZE_T) :: slen
108 type(C_PTR) :: p
109 p = fftw_export_wisdom_to_string()
110 if (.not. c_associated(p)) stop 'error exporting wisdom'
111 slen = strlen(p)
112 call c_f_pointer(p, s, [slen+1])
113 ...
114 call free(p)
115 </pre></div>
116 <a name="index-c_005fassociated"></a>
117 <a name="index-c_005ff_005fpointer-3"></a>
118
119 <p>Note that <code>slen</code> is the length of the C string, but the length of
120 the array is <code>slen+1</code> because it includes the terminating null
121 character. (You can omit the &lsquo;<samp>+1</samp>&rsquo; if you don&rsquo;t want Fortran to
122 know about the null character.) The standard <code>c_associated</code> function
123 checks whether <code>p</code> is a null pointer, which is returned by
124 <code>fftw_export_wisdom_to_string</code> if there was an error.
125 </p>
126 <a name="index-fftw_005fimport_005fwisdom_005ffrom_005fstring-1"></a>
127 <p>To import wisdom from a string, use
128 <code>fftw_import_wisdom_from_string</code> as usual; note that the argument
129 of this function must be a <code>character(C_CHAR)</code> that is terminated
130 by the <code>C_NULL_CHAR</code> character, like the <code>s</code> array above.
131 </p>
132
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